GaAlAs light-emitting diodes (LEDs) are a popular optical source for optical communication applications using optical fibers. The optical output is typically directly pulse modulated by varying the driving current. For binary (2-level) digital applications, a current in the range 50 to 300 mA must be switched on and off at high speed through the LED in response to a low-level data-input signal. A small dc forward bias (approximately 1mA) added to the switched current may prove advantageous in high-bit-rate appications by maintaining charge on the diode's capacitance. The intrinsic radiative recombination time of the LED permits very high modulation rates.
For an ideal LED, if the injected carriers arrive instanteously at the recombination (diffussion) region, the rise time of the spontaneous emission is governed solely by the spontaneous recombination time of the carriers. However, in driving a practical diode, the junction capacitance and the stray capacitance cause delay in the arrival time of the injected carriers at the recombination region. Thus, the rise time of the spontaneous emission would be either (i) material-limited by the spontaneous recombination time or (ii) circuit-limited by the time constant of the driving circuit (including the junctional capacitance of the diode).
For typical LEDs such as Hewlett Packard's HFBR-1402 the optical pulse risetime attainable when (i) is the limiting factor is a small fraction of that attainable when (ii) is the limiting factor.
One way to minimize the effect of the junction and stray capacitance is to use a low impedance (voltage type) driver. This is undesirable because the resulting LED current and therefore the optical flux would then not be accurately controlled. Low impedance drivers have been used to switch LEDs. A survey of these drivers is contained in "Semiconductor Devices for Optical Communications", Topics in Applied Physics, Vol. 39, pp. 170-182. The problem is the difficulty in controlling the LED current and, therefore, the optical power. The drivers described therein are unsatisfactory at high bit rates because the phase shift and gain provided are unsuitable and circuit stability is difficult to obtain.
Analog compensation of LED junction capacitance such as is described in "Drive Fiber-Optic Lines at 100 MHz", Electronic Design, Vol. 15 (1974) pp. 96-99 is another technique. In this class of implementations the LED is operated in its "linear" region. A filter is used to decrease attenuation with frequency in order to compensate for the roll-off in the LED frequency response. The over-all frequency response is thus flat over a large frequency range. One problem associated with this technique is that the LED is only linear over a small region and consequently one cannot take advantage of the full power output of the LED. Another problem is that the LED capacitance varies significantly from device to device and consequently most LEDs will be significantly over/under compensated.